In prior U.S. Pat. No. 4,360,728, assigned to the assignee of the present invention, a banking card is disclosed suitable for use with automatic teller machines (ATMs). The patent describes a card having optical data written by a laser which forms pits in the medium. The medium itself is described as a thin metallic recording layer made of reflective metal. Types of reflective recording material suitable for use in such cards are described in U.S. Pat. Nos. 4,269,917; 4,304,848 and 4,363,870, all assigned to the assignee of the present invention. This material is characterized by reflective silver particles in a gelatin matrix. The silver particles form a reflective field which may be modified by laser writing or, in some instances, by photographic prerecording of information. Since the material described in the patents is based upon silver halide emulsions, exposure of data patterns and subsequent developing of the patterns leads to recording of data in a manner similar to laser writing. In either case, data is represented by spots or pits in a reflective field. Reflective spots in an absorptive field would also work.
In many optical data recording and retrieval systems, data has been recorded in linear tracks, analogous to magnetic recoding. Data is read by following tracks, usually by means of servo systems. Some lengthwise compression of data has been achieved in both magnetic and optical systems by means of clever modulation schemes, such as FM and MFM. Using such techniques, errors are reduced, while at the same time data bits may be more closely spaced. In U.S. Pat. No. 3,920,957 an optical data modulation scheme is disclosed for bar code which eliminates space in isolating individual bits of data. The elimination of such space between adjacent bits can double the density of recorded bits in the lengthwise direction.
An object of the present invention was to devise a means for formatting optically recorded data for a data card, or the like, in a way that increases data density, while at the same time minimizes error.